1. Field of the Invention
The invention is related to a method for automatically adjusting an optical axis of a laser resonator.
2. Description of the Prior Art
In recent years, the fields in which lasers are applied have been expanding dramatically. Among these lasers, the helium-cadmium or He-Cd laser (main oscillation wave length: 325 nm) is expected to be used in various fields such as: a source of light to expose photo-resist resin, a source of light to cure photo-curing resin when models and metallic molds are designed by means of the photo-curing resin, a source of light to cure photo-curing resin when models of human frames and bones are made using the data of computed tomography, a source of light for the fluorometric analysis, and so on.
The mirrors in a laser resonator, except the He-Ne laser of the small internal mirror type and a semiconductor laser device, are usually held by three or four rods each having a small coefficient of thermal expansion, such as an iron alloy (Fe 64% . Ni 36%). The optical axis of the mirrors is designed to be kept stable by these rods.
The optical axis of such mirrors changes, however, because a temperature gradient is produced by heat generated inside a laser head when the laser equipment is activated and the residual strain of the laser resonator changes with the passage of time. This change of the optical axis of the mirror leads to variations in laser output, subsequently causing a variety of problems. Furthermore, the laser action must be stopped for some time in order to re-adjust the optical axis.
At present, mirror angle adjustment is made manually by means of the following equipment. Namely, a fixed plate is fixed to the rods. The mirror is fixed to a movable plate which is pulled to the fixed plate by a tension coil spring. Further, adjustment bolts for the mirror angle are screwed in, for example, at two corner parts of the movable plate, and the ends of the bolts are put in contact with the fixed plate. The leakage beam from the reflection mirror of the laser or the output beam and so on are separated into spectral components and detected. While the change of the detection intensity is monitored, the adjustment bolts for the mirror angle are manually screwed to maximize the output intensity of the laser. In this regard, see U.S. Pat. No. 3,897,139 issued to Caruolo et al.
However, the above-mentioned method of manual adjustment takes an inordinate amount of time to properly adjust the mirror angle to maximize the output of the laser. Even after the output of the laser is adjusted properly, the mirror angle changes from the optimal value in a short time, hence requiring repeated cumbersome adjustment operations. This is both disadvantageous and inconvenient.
On the other hand, in recent years piezo-actuators and linear actuators operated by means of direct current (DC) motors have been miniaturized and are available with high performance capabilities. Therefore, these actuators have been considered for application in lasers to automatically adjust the mirror angle.
Under the present conditions, however, the adjustment of the mirror angle by means of these actuators is only a technique at a laboratory level, and no practical and economical methods have been established.
Namely, in Japanese Laid-Open Patent Publication Number 3-35579 (1991), a technique is disclosed which converts an analog signal of a laser beam for the measurement into a digital signal by means of an analog-to-digital converter, compares the successive digital signals with each other by means of a program stored in a Read-Only memory, and detects and adjusts the maximal power. However, this technique requires an expensive analog-to-digital converter, a micro-computer and software. While it may be possible to use this technique in conjunction with expensive laser equipment, such as carbon dioxide gas laser equipment, it is not an economical solution for use with ordinary popular laser equipment because of the high cost of the associated equipment used in this technique given the performance that will result therefrom.